ci: fix misleading mysql57 label on ci-mysql84-g{1..5} check runs

The five ci-mysql84-g*.yml reusables introduced in #5597 inherited
`infradb: [ 'mysql57' ]` from ci-legacy-g4.yml as the sed template.
The `infradb` matrix value is interpolated into `env.MATRIX` and then
into the check-action step's `name` field, so the GitHub PR UI would
display check runs as `CI-mysql84-g3 / tests (mysql57)` even though
the tests themselves are correctly running against infra-mysql84.

The infrastructure resolution is unaffected -- `ensure-infras.bash`
strips the -gN suffix from TAP_GROUP=mysql84-g3 to get BASE_GROUP=mysql84,
which resolves to test/tap/groups/mysql84/infras.lst (infra-mysql84).
This is purely a display-name fix.

Set infradb to 'mysql84' in all five files so the check run name
matches the backend actually under test.

Caught by gemini-code-assist review on #5598.

.github/workflows/ci-mysql84-g1.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        infradb: [ 'mysql57' ]
+        infradb: [ 'mysql84' ]
     env:
       BLDCACHE: ${{ inputs.trigger && fromJson(inputs.trigger).event.workflow_run.head_sha || github.sha }}_ubuntu22-tap_src
       MATRIX: '(${{ matrix.infradb }})'

.github/workflows/ci-mysql84-g2.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        infradb: [ 'mysql57' ]
+        infradb: [ 'mysql84' ]
     env:
       BLDCACHE: ${{ inputs.trigger && fromJson(inputs.trigger).event.workflow_run.head_sha || github.sha }}_ubuntu22-tap_src
       MATRIX: '(${{ matrix.infradb }})'

.github/workflows/ci-mysql84-g3.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        infradb: [ 'mysql57' ]
+        infradb: [ 'mysql84' ]
     env:
       BLDCACHE: ${{ inputs.trigger && fromJson(inputs.trigger).event.workflow_run.head_sha || github.sha }}_ubuntu22-tap_src
       MATRIX: '(${{ matrix.infradb }})'

.github/workflows/ci-mysql84-g4.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        infradb: [ 'mysql57' ]
+        infradb: [ 'mysql84' ]
     env:
       BLDCACHE: ${{ inputs.trigger && fromJson(inputs.trigger).event.workflow_run.head_sha || github.sha }}_ubuntu22-tap_src
       MATRIX: '(${{ matrix.infradb }})'

.github/workflows/ci-mysql84-g5.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        infradb: [ 'mysql57' ]
+        infradb: [ 'mysql84' ]
     env:
       BLDCACHE: ${{ inputs.trigger && fromJson(inputs.trigger).event.workflow_run.head_sha || github.sha }}_ubuntu22-tap_src
       MATRIX: '(${{ matrix.infradb }})'