Document the support policy for minimum required GMT versions

.github/ISSUE_TEMPLATE/release_checklist.md

@@ -16,7 +16,7 @@ assignees: ''
 - [ ] Wrap Y ()
 
 **Before release**:
-- [ ] Check [SPEC 0](https://scientific-python.org/specs/spec-0000/) to see if we need to bump the minimum supported versions of Python and core package dependencies (NumPy/Pandas/Xarray)
+- [ ] Check [SPEC 0](https://scientific-python.org/specs/spec-0000/) to see if we need to bump the minimum supported versions of GMT, Python and core package dependencies (NumPy/Pandas/Xarray)
 - [ ] Run `make codespell` to check common misspellings. If there are any, either fix them or add them to `ignore-words-list` in `pyproject.toml`
 - [ ] Check to ensure that:
   - [ ] All tests pass in the ["GMT Legacy Tests" workflow](https://github.com/GenericMappingTools/pygmt/actions/workflows/ci_tests_legacy.yaml)

README.md

@@ -193,11 +193,15 @@ Other non-official Python wrappers for GMT (not maintained):
 
 ## Minimum supported versions
 
-PyGMT has adopted [SPEC0](https://scientific-python.org/specs/spec-0000/) alongside the rest of the
-Scientific Python ecosystem, and therefore:
+PyGMT has adopted [SPEC 0](https://scientific-python.org/specs/spec-0000/) alongside the
+rest of the Scientific Python ecosystem, and therefore:
 
 - Support for Python versions be dropped 3 years after their initial release.
-- Support for core package dependencies (NumPy/Pandas/Xarray) be opped 2 years after their initial release.
+- Support for core package dependencies (NumPy/Pandas/Xarray) be dropped 2 years after
+  their initial release.
 
-For the supported GMT versions as well as the minimum required versions of Python and core package
-dependencies, please see [Minimum Supported Versions](https://www.pygmt.org/dev/minversions.html).
+Similarly, the PyGMT team has decided to discontinue support for GMT versions 3 years
+after their initial release.
+
+Please see [Minimum Supported Versions](https://www.pygmt.org/dev/minversions.html) for
+the minimum supported versions of GMT, Python and core package dependencies.

doc/maintenance.md

@@ -119,15 +119,20 @@ to change the default behaviour at <https://docs.readthedocs.io/en/stable/config
 
 ## Dependencies Policy
 
-PyGMT has adopted [SPEC 0](https://scientific-python.org/specs/spec-0000/) alongside
-the rest of the Scientific Python ecosystem, and therefore:
+PyGMT has adopted [SPEC 0](https://scientific-python.org/specs/spec-0000/) alongside the
+rest of the Scientific Python ecosystem, and therefore:
 
 * Support for Python versions be dropped 3 years after their initial release.
-* Support for core package dependencies (NumPy/Pandas/Xarray) be dropped 2 years after their initial release.
+* Support for core package dependencies (NumPy/Pandas/Xarray) be dropped 2 years after
+  their initial release.
+
+Similarly, the PyGMT team has decided to discontinue support for GMT versions 3 years
+after their initial release.
 
 In `pyproject.toml`, the `requires-python` key should be set to the minimum supported
-version of Python. Minimum supported versions of Python and core package dependencies
-should be adjusted upward on every major and minor release, but never on a patch release.
+version of Python. Minimum supported versions of GMT, Python and core package
+dependencies should be adjusted upward on every major and minor release, but never on a
+patch release.
 
 
 ## Backwards Compatibility and Deprecation Policy

doc/minversions.md

@@ -7,6 +7,9 @@ rest of the Scientific Python ecosystem, and therefore:
 - Support for core package dependencies (NumPy/Pandas/Xarray) be dropped 2 years after
   their initial release.
 
+Similarly, the PyGMT team has decided to discontinue support for GMT versions 3 years
+after their initial release.
+
 | PyGMT Version | GMT | Python | NumPy | Pandas | Xarray |
 |---|---|---|---|---|---|
 | [Dev][]* [[Docs][Docs Dev]] | >=6.3.0 | >=3.10 | >=1.23 | >=1.5 | >=2022.03 |

pygmt/clib/session.py

@@ -149,7 +149,7 @@ class Session:
     -55 -47 -24 -10 190 981 1 1 8 14 1 1
     """
 
-    # The minimum version of GMT required
+    # The minimum supported GMT version.
     required_version = "6.3.0"
 
     @property